This invention relates to a tightening system and/or a method of tightening.
Threaded fasteners are conventionally tightened on an assembly line using air driven motors supplied from a central compressed air facility. The fastener may be tightened by any of a number of methods including torque control, angle control, tension or yield control, removal of the air supply to the motor, or using a solenoid valve, at the required stopping point. The major advantage of air motors for this application are their low cost, ruggedness, reliability, and safety. The safety feature is particularly important as expanding air in the motor cools the motor and therefore the fire hazard resulting from overheating motors is minimized.
However notwithstanding this major advantage of air driven tools air compressors are both inefficient and expensive to maintain while air motors are even less efficient and compressed air plumbing is likewise expensive to maintain. The overall efficiency of an air motor drive system is typically 3% (from electrical power in to mechanical power out) and higher efficiencies are not possible without higher expansion ratios in the air motors leading to dangerously low motor temperatures which may be hazardous to personnel.
From the control point of view, air motors are also poor as in principle they can really only be turned on or off. Air motor rotors are typically high inertia so that stopping points are often poorly defined and relatively large over-runs are common. In addition the on/off control is usually exercised by solenoid valves which may have 10-30 ms response times leading to larger over-runs still. Reduced speed tightening is in principle possible using flow control valves but in practice it is even less efficient, gives poor control, and so is not used.
In large assembly plants with several hundred air motors low efficiency operation represents a jafor cost item and high efficiency drives are sought after. Both AC and DC electric motors have been used for this purpose by a variety of manufacturers and offer efficiencies of perhaps 65% or more. DC motors are by far the easiest to control and have achieved a measure of success but are unlikely to gain total acceptance as they require regular maintenance of their commutators. In the particular application envisaged here, bolt tightening, the duty cycle may be small but the final tightening inevitably corresponds to a considerable over-rating of the motor. This momentary over-rating is essential to keep the physical size of the motor down so that the inter-spindle spacing can be kept as small as possible. Pulse currents 5 to 10 times full rated current cause greatly reduced brush lives to the point where the motors are unacceptable in high volume production.
A new type of AC motor has begun to gain acceptance in these application that is the brushless DC motor. In essence this motor is an inverted DC motor with an electronic communtator and a permanent magnet rotor. For very high torque and power to size ratios expensive samarium cobalt magnets are often used to give very high field strengths. These magnets cannot be degaussed so that extreme electrical conditions can now be tolerated but the magnets are a ceramic material which must be glued to the rotor and the glue bond is often the weak link. Motor disassembly is a skilled job as the air gaps are small and magnetic keepers must be used to stop the rotor from being attracted to the stator. If it is the force of impact is likely to shatter the magnets--note that motors with Alnico magnets also require keepers but for a different purpose: i.e. to stop the Alnico from being degaussed.
The very high field strengths achieved by the Samarium cobalt magnets means that the back end of the motor (i.e. the terminal voltage self generated when the motor is spinning) is high at high speeds so that high voltage electronic commutators (typically 300 volts) are required. These commutators must be controlled by a form of shaft encoder or resolver to achieve the correct switching sequence in the electronics.
In summary therefore brushless CC motors are expensive, require high voltages, and need a number of angle sense wires between the motor and the electronics. They are therefore potentially hazardous and require highly trained maintenance personnel.
An alternative AC motor, i.e. the induction motor,--has been known of for many years and is conveniently driven from a fixed frequency supply. To minimize the motor size "high-cycle" motors operating at 200 to 300 Hz with appropriate gearing are often used. These motors have high inertia giving poor stopping control, require high hazardous voltages to give full range performance, and under sustained overloads they are a potential fire hazard as they will rapidly overheat.